Synchronized communications system



SYNCHRONIZED COMMUNICATIONS SYSTEM Filed Dec. 6, 1965 17 Sheets-Sheet 1 FIG. 1

SYSTEM TIMING FOR SYNCHRONIZATION m IIIEEIIII STAR; PULSE "I POSITION NO. 5 4

200 547 1 TIME, SECONDS 548 548 w m Ix RX STATION x l I u (TRANSMIT) I y STATION Y I b (TRANSMIT) STATION x I II c (RECEIVE) I 1X RX STATION Y I H d (RECEIVE) STATION x lo MEASURES 8 STATION Y MEASURES 20 f to I(I t2 SYNCHRONIZATION BETWEEN TWO STATIONS (IN SYNC. CONDITION) INVENTOR. WALTON GRAHAM ATTO R N EYS July 19, 1966 w. GRAHAM 3,262,111

SYNCHRONI ZED COMMUNICATIONS SYSTEM Filed Dec. 6. 1963 17 Sheets-Sheet 2 x 3 SYNcI-IRONIzATION BETWEEN TWO STATIONS (OUT-OF-SYNC. CONDITION) IX Rx STATION x H q (TRANSMIT) y y STATION Y T H b (TRANSMIT) y y STATION x T T c RECEIVE 7 1X R STATION Y 1 d (REcEIvE) STATION x |3 V MEASURES 20 e STATION Y 20 f MEASURES o o I HM 2 tzwt TO INTERROGATION INTERROGATION PULSE GATE INTERROGATION PHASE PULSE OSCILLATOR SHIFITERL GENERATOR AND TRANSMHTER I I4 II I I0 REPLY (R) I PUIsE K IN 3 RPY T IS 4 l9 1, LOW PASS I F|| TER ERPY EINT INTERRO- I3 GATION (I) PLllhsE INT INT INVENTOR. WALTON GRAHAM ATTORNEYS July 19, 1966 w. GRAHAM 3,262,111

SYNCHRONI ZED COMMUNICATIONS SYSTEM Filed Dec. 6. 1963 17 s s 5 S= NUMBER OF COMPARISONS REQUIRED TO REACH SYNCHRONIZATION 2 4 0 l0 I00 IOOO 10,000 100,000

5 +A'STATION Y STATION 2 sTAT|0N x FIRST INTERVAL SECOND INTERVAL \?x-| g x-z STATION x c TRANSMITS B fix-z STATION Y T1 b RECEIVES STATION Y J1 C TRANSMITS i [4 ma STATION 2 d RECEIVES BM STATIONZ M e TRANSMITS COARSE SYNCHRONIZATION 6 USING START (B) PULSES INVENTOR. WALTON GRAHAM BY 00 0M,

ATTORNEYS July 19, 1966 w. GRAHAM 3,262,111

SYNCHRONIZED COMMUNICATIONS SYSTEM Filed Dec. 6, 1963 17 Sheets-Sheet 4 {STATION Y 111057 \4\STA}'ION x e IX RXZ X H l l I I I l I I I I II I I I l l I Lu (TRANSMITS) 5 2 l0 I5 20 sTATIoN z (TRANSMITS) I2 sTATIoN x J c (RECEIVES) STATION 2 (RECEIVES) n n d L /FIG. 7B IY STATIONY I I III (TRANSMH'S) s I: RZY Io I5 20 STATION 2 (TRANsMITs) STATION Y JJ c (RECEIVES) IY sTATIoN 2 L (RECEIVES) ATlON x I l (TRANsMITsI IY 5 RYZIO I5 20 sTATIoN Y (TRANSMITS) I RZY STATION Z (TRANSMlTS) 2 YZ ZY (STATION M d RECEIVES X Iz Y Y AII E'III SI J U H 6 I I \(z xY STATION 2 IT I! J H f L (RECEIVES) INVENTOR. WALTON GRAHAM ATTOR N EYS July 19, 1966 W. GRAHAM Filed Dec. 6, 1963 FIG. 8

17 Sheets-Sheet SYSTEM TIMING DURING DATA TRANSMISSION PERIOD BASE BASE sTATIoN A sTATIoN aw F 1 #1 P0sITI0N*2 3 4 TIME o 440 44OSECONDS RANGE o 425 850 MILES R TRANSMITTER SWITCH REcEIvER I3 I50 r' I 4 I %AJ1A PULSE SYSTEM GENERATORS MIN T DECODERS I36 I30 I32 DATA L f IN GATES SYNCHRONIZED SYNCHRONIZING oscII I AToR CIRCUITS B PULSE J u PULSE POSITIONS 1 b PHANTASTRON H c OUTPUT PosITIoN PULSE d L A 0R G PULSE e GATE OUTPUT J f INVENTOR.

WALTON GRAHAM ATTORNEYS July 19, 1966 w. GRAHAM SYNCHRONIZED COMMUNICATIONS SYSTEM y 9, 1966 w. GRAHAM SYNCHRONIZED COMMUNICATIONS SYSTEM 1'7 Sheets-Sheet 8 Filed Dec. 6, 1963 an? .5056 woz wtqw zombwfizm y $1 mml mm; a; v9 Flllllllllll'lllllllll i l l I lllllllll-U M 8 M M a 0 mm M E6 Jr W V mm M m u A w fi w fiw 5o om 2055 950% x0595? we??? MEG MES. m v lim llL 8 July 19, 1966 w. GRAHAM 3,262,111

SYNCHRONI ZED COMMUNICATIONS SYSTEM FIG. 13A

SYNCHRONIZED POSITION FOR RECEIPT OF OTHER Filed Dec. 6, 1963 17 Sheets-Sheet 9 OWN I PULSE RECEIvED OWN INTERR REPLY INTERR. POSITION PULSE POSITION l A I ERROR 4 T I T I POSITIO POSITION RECEIVED 20L. OTHER t INTERR. PULSE SUBTRACT RE C'CEINT COUNT SUBTRACT COUNT (FREQUENCY OF TIMING PULSES QUADRUPLED) COUNT K IN SYNC. t 2 COUNTER a 7 CORRECT PULSE 3 CORRECTED OWN INTERR. POSITION NO TIMING PULSES OWN TO POSITION INTERR. COUNTER 220 A POSITION /2 CORRECTED 4 POSITIONS I t SYNCHRONIZATION LEAD CASE INVENTOR.

WALTON GRAHAM BY pM M/ ATTO RN EYS July 19, 1966 W. GRAHAM SYNCHRONIZED COMMUNICATIONS SYSTEM Filed Dec. 6, 1963 FIG 1'? Sheets-Sheet 10 I PULSE OWN RECEIVED OWN INTERR. REPLY INTERR. POSITION PULSE POSITION I T T 5 I wex INTERR 2f PULSE SUBTRACT INCREASE\ COUNT COUNT ZERO gmc IN [CROSSING 2 COUNTER /ADD COUNT wraa ms QUADRUPLED) t CORRECT PULSE 3 t DOUBLE FREQ. N INTERR TIMING PULSES fifi EE%T%% POSITION TO POSITION A COUNTER 220 3 CORRECTED l 4 POSITIONS a! A. t

SYNCHRONIZATION LAG CASE INVENTOR.

WALTON GRAHAM BY 03% MM ATTORNEYS July 19, 1966 w. GRAHAM 3,262,111

SYNCHRONIZED COMMUNICATIONS SYSTEM Filed Dec. 6, 1963 17 sheetsfiwet 15 CLEAR CLEAR OTHER N PULSE PULSE TERROGA 272 PULSE I I I I I BISTABLE AND BISTABLE DELAY FLIP- FLOP GATE I P 2T4 DUMP VOLTAGE REPLY DUMP CIRCUIT PULSE I289 A/C POSITION GATE PULSE A0 BASE STATION GATE POSITION PULSE 0 292 I INFO] SYNC.

INFO- SYNC. INVERTER COUNTER 279 DELAY POSITION PULSES FROM COUNTER 220 (FIG. I4)

DELAY GATE 2 START POSITION PULSE (TO FIG. 2a)

PULSE POSITION SELECTOR FOR B,A ANDG PULSES INVENTOR. WALTO N G RA HAM ATTORNEYS July 19, 1966 W. GRAHAM Filed Dec. 6, 1963 17 Sheets-Sheet 14 "OWN" INTERROGATION POSITION PULSE SELECTOR 300 304 I I INTERROGATION POSITION POSl(" l; l8I|: |GPSUl@SE23 PULSES FROM GATE GATE I a I COUNTER 22o H IT| I I INFOI SYNCI 307x I FROM 292 I BLOCKING RAN DOM OSCILLATOR PERIOD 308 309 B (FROM 486 OF FIG.24)B PULSE DIGITAL TO 7 THRESHOLD 3IO couNTER ANALOG BO (RESET) NOISE GENERATOR s05 FROM 287OF 320 FIG. I7) RECEIVE D 328 326 DECODED sTART PULSES v AND A AND BISTABLE CLOSE GATE GATE FLIP-FLOP GATE 2T2 AND REsET couNTER 22o [INFO ISYNcI FIG. l4)

F(ROM 2s a2 FIG. I?

I RESET I 522 324 DIFF. BISTABLE AND d/dT FLIP-FLOP GATE FIG. 20

CORRECT PULSE FROM 209 (FIG. l4)

4OO CPS START PULSE COARSE SYNCHRONIZATION ZEROCROSS VOL GE IFROM FLIP-FLOP 225 FIG. I4) 356 FREQUENCY CORRECTION VOLTAGE MASTER TIMING OSCILLATOR INVENTOR.

FREQUENCY CONTROL WALTON GRAHAM ATTORNEYS July 19, 1966 w. GRAHAM 3,262,111

SYNCHRONI ZED COMMUNICATIONS SYSTEM Filed Dec. 6. 1963 17 sheetsheet l7 TRANSM TTI G RECElVING ANTEBINAN ANTENNA FROM PULSE CODER (FIG. 23)

VARACTOR SWITCH SMALL DELAY MODULATOR LOCAL i jj OSC. AIMER A.\/. C.

TIME 514 $615 VARIABLE GEN. 52o s|e 5|8 VIDEO AMF. RECEIVER TRANSMITTER L TO VIDEO DECODER (FIG. 24)

INVENTOR.

WALTON G R AHAM ATTOR NEYS United States Patent C) 3 262,111 SYNCI-RONIZED CdMMUNICATIONS SYSTEM Walton Graham, Roslyn, N.Y., assignor, by mesne assignments, to Control Data Corporation, South Minneapolis, Minm, a corporation of Minnesota Fiied Dec. 6, 1963, Ser. No. 328,655 46 Claims. (Cl. 343-75) This invention relates to radio communications systems and more particularly to a system for providing range, bearing and altitude information for a plurality of stations with respect to each other in order to provide for navigation, collision avoidance and air trafiic control capabilities.

I. INTRODUCTION In applicants prior copending applications Serial No. 35,659 filed June 13, 1960, entitled A Compatible Airborne Navigation-Air Traffic Control and Collision Avoidance System now US. Patent No. 3,183,504, and Serial No. 42,886, filed July 14, 1960, and having the same title, both of which are assigned to the assignee of this application, systems were disclosed which provided navigation, collision avoidance and air trafiic control capabilities for a plurality of stations. The systems previously disclosed utilize a master station which transmits synchronizing signals to a number of fixed base stations. The fixed base stations, which are at a known distance from the master station and therefore have a known time delay between the transmission and reception of the synchronizing signals, use these synchronizing signals to become synchronized with the master station, thereby providing a network of synchronized base stations.

Each of the base stations in those systems transmits reference pulses at a fixed rate and on a different carrier frequency. These reference pulses are received by a number of movable stations, such as aircraft. The movable stations transmit interrogation pulses to which the base stations respond by transmitting reply pulses. Enough different carrier frequencies are used to permit unambiguous interrogation of and reply by a particular base station.

The movable stations measure their range to any 'base station within their transmission range using beacon techniques which includes the transmission of interrogation pulses by the movable station combined with an automatic search for the reply pulse transmitted by the base station in response to a particular movable stations interrogation pulse. The search technique is used in order to separate the desired reply pulse from the reply pulses transmitted by the base station in response to interrogation pulses from other movable stations. Since each movable station can determine its range to a base station, thereby knowing the time of propagation of the base station reference pulses, it is possible to synchronize the interrogation pulses of the movable stations with the base station reference pulses. Therefore, since every movable station is synchronized with one or another base station, all of the base stations being synchronized with each other, all of the movable stations are in synchronism. Consequently, every movable station is capable of measuring the range to every other movable station or base station by observing the time of arrival of pulses from those stations. By restricting the time of transmission of various ones of the pulses from the moving stations to certain transmission positions, it is possible to provide additional information concerning the movable station, such as its altitude. Also, it is possible to make measurements on variou received pulses in order to determine the bearing of one station from another. Therefore, these previous systems provide a complete arrangement having navigation, collision avoidance and air traffic cntrol capabilities.

While the aforesaid systems provide a complete working arrangement for the desired operations, several disadvantages :are present. First of all, the presence of a number of base stations is required and these base stations must be synchronized with each other at the added cost of providing an auxiliary system for synchronization. Additionally, the operation of the various base stations on diiferent frequencies requires the use of a substantial radio frequency bandwidth out of the already crowded frequency spectrum and also requires transmitters and receivers for the movable stations which must be both stable in frequency and tunable over the frequency band. Since the receivers for the movable stations must have a bandwith which is adequate to receive interrogation pulses transmitted at many possible frequencies in a wide frequency band, the range of operation of the systems is limited by the design of these wide band receivers.

The present invention is directed to a system for navigation, air trafiic control and collision avoidance which eliminates many of the aforesaid disadvantages and also introduces unique operating advantages. In the present system, no master station is needed and there are no intermediate base stations which must be synchronized with each other by some auxiliary means. Instead, every station acts to synchronize with every other station, whether the station is fixed or movable. Also, all transmissions and receptions from movable or fixed stations occur at the same frequency, thereby considerably reducing the cost of each stations transmitter and receiver and'at the same time allowing for greater receiver sensitivity and operation of the transmitters at relatively low powers. One advantage of the present system is that each base station is essentially identical with the movable stations and each base station is capable of providing range information to any of a number of movable stations by transmission of a small, fixed number of additional pulses, no matter how movable stations without the necessity of any base sta-' tion. This is desirable for providing collision avoidance and [other necessary information to movable stations which are operating over large areas, for example, oceans,

where base station facilities are unavailable.

It is therefore an object of this invention to provide a navigation, air traffic control and collision avoidance system.

A further object of this invention is to provide a system for synchronizing the transmissions of various stations, both fixed and mobile, without the use of separate synchronization apparatus.

Yet another object of the invention is to provide a navigation, air trafiic control and collision avoidance sysstem in which the transmissions of the fixed and mobile stations are synchronized.

Still a further object of the invention is to provide a system for synchronizing the transmissions of various stations, both fixed and mobile, and using these transmissions to provide range and altitude information of one station with respect to another.

Another object of this invention is to provide a system for synchronizing the transmissions of a plurality of stations by using only the transmissions themselves.

Other objects and advantages of the present invention will become more apparent upon reference to the following specification and annexed drawings, in which:

FIGURE 1 is a timing diagram showing various transmissions from base and movable stations;

FIGURE 2 is a timing diagram showing the transmitted and received pulses of two stations in the synchronized condition;

FIGURE 3 is a timing diagram showing the transmitted and received pulses for two stations in the unsynchronized (out-of-sync) condition;

FIGURE 4 is a schematic block diagram of one type of circuit for correcting the phase of an oscillator to obtain synchronization;

FIGURE 5 is a graph showing the number of comparisons between stations required to reach synchronization;

FIGURE 6 is a timing diagram showing the use of START (B) pulses to achieve coarse synchronization between stations;

FIGURES 7A, 7B and 7C show the effects of coarse synchronization on three stations;

FIGURE 8 is a timing diagram illustrating certain operating principles of the invention;

FIGURE 9 is a simplified block diagram of the system of the invention;

FIGURE 10 is a detailed block diagram of typical components which may be used in the system of the present invention;

FIGURES 11A, 11B and 11C are schematic block diagrams of portions of the system of the present invention;

FIGURE 12A is a schematic block diagram of the altitude gating circuit;

FIGURE 12B is a timing diagram showing the operation of the altitude gating circuits;

FIGURES 13A and 13B are timing diagrams showing correction for synchronization using a pulse counting technique;

FIGURE 14 is a block diagram of the equipment at a station for producing synchronization using pulse counting;

FIGURES 15A and 15C are block diagrams of different types of reversible counters while FIGURE 15B illustrates binary counting techniques;

FIGURE 16 is a block diagram of a circuit for producing the dump voltage;

FIGURE 17 is a block diagram of a circuit for selecting the position pulses for the B A and G pulses;

FIGURE 18 is a block diagram of a circuit for selection of position pulses for the I pulses;

FIGURE 19 is a block diagram of a circuit for producing coarse synchronization of the start pulses;

FIGURE 20 is a frequency control circuit for the master timing oscillator of FIGURE 14;

FIGURE 21 is a block diagram of a circuit for selecting posit-ion pulses to produce the A pulses;

FIGURE 22 is a block diagram of a circuit used to prevent production of information pulses when the station is unsynchronized;

FIGURES 23 and 24 are block diagrams of a pulse coder and decoder, respectively; and

FIGURE 25 is a block diagram of a receiver-transmitter for use with the system.

II. SYSTEM SIGNAL TRANSMISSIONS In order to explain the operation of the system of the present invention, the foliowing symbols are adopted for the various transmissions:

R-reply pulses Iinterrogation pulses B-start pulses Gground station information pulses Aairborne station information pulses 0subscript designating a signal transmitted by own station Throughout the description, the term base station is used to mean a station which is fixed relative to the movable stations. The base station, for example, may be a fixed ground station or a relatively stationary beacon station operating on the water. The term movable station is used to define those stations which move relative to the base stations and/or to each other. These may be, for example, aircraft, helicopters, or other types of stations moving in the air, on the ground, or on the water. It should be realized that other stations will fall within the definition of base or movable in the manner as defined herein.

In order to explain the operation of the system, reference is made to FIGURE 1 which shows some of the signals that are transmitted during each operating interval which, for the purposes of explanation is assumed to be one second.

The one second interval is divided up illustratively into 548 transmission positions occurring every second. At t equal 0 second and at transmission position #1 the transmission interval begins with a START pulse B which is transmitted by each operating station. Positions #2 to #200 are data transmission positions with the even numbered positions, #2, #4 #200 assigned to particular base stations for transmission of base station information pulses G and the odd numbered positions starting at #3 and up to #199 assigned to the movable stations. In the illustrative system application being described, the odd numbered positions are for transmission of airborne station information pulses A in accordance with the altitude layer within which the respective airborne stations are located.

For example, with respect to the base stations, base station X is assigned to transmit an information pulse G at position #2, base station Y to transmit an information pulse G at position #4 and base station ZZ to transmit information pulse G at position #200. Since a particular base station transmits an information pulse only at its assigned transmission position with respect to start pulse B, a base station may therefore be identified by its transmission position. It should be realized that the same transmission position can be used for two base stations provided the two base stations are sutficiently far enough apart, so that the transmissions from one base station cannot be received by a movable station operating within range of the other base station.

In the illustrative embodiment of the invention being described, the odd numbered positions #3 #199, for transmission of aircraft information pulses, correspond to a plurality of successive altitude layers of 300 feet. Therefore, those aircraft in the altitude layer from 0300 feet transmit information pulses A at position #3, those aircraft in the altitude layer from 300600 feet transmit pulses A in position #5, and so forth. It should be realized that a plurality of aircraft may be located in a particular altitude layer and each of these aircraft will transmit its respective altitude information pulse only at the proper position. The height of the altitude layers can be established in accordance with the complete system requirements, which includes the operating altitudes of the various aircraft. It should also be realized that each altitude layer does not have to be the same height but, for example, the height of the respective altitude layers can be increased with increasing altitude in order to take into consideration the fact that the accuracy of aircraft altimeters decreases with increasing altitude. In this case, therefore, the upper altitude layers would be of greater height than the lower altitude layers. For example, the lower layers would be 300 feet each and the upper layers 1000 to 1500 feet each.

Transmission positions #201 to #548 of FIGURE 1 are used by both the base stations and the movable stations to transmit interrogation pulses I for the purpose of obtaining and maintaining synchronization among all of the stations. The achievement of synchronization is necessary in order that the various stations may be able to transmit information at the proper positions and identify all the positions to obtain information from the other stations. According to the principles of the invention, no station, fixed or movable, transmits information pulses G or A in positions #2 to #200 unless it is synchronized. The determination of synchronization condition is automatically recognized by each station by the production of zero or minimal error signals in the stations error determining circuits and synchronization is achieved in a manner to be described.

Once the various stations are synchronized, they are able to determine the range to and altitude of the other stations within their transmission and reception range. For example, upon obtaining synchronization each movable station knows the transmission position and time of an information pulse G from a particular ground station and the measurement of the time between the transmission position and the reception of the G information pulse gives the range from the movable station to the ground station. A ground station is able to determine the range and altitude of aircraft by measuring the time of reception of the aircraft information pulse A after the occurrence of an odd numbered position and noting the transmission position number. In the same manner, each aircraft can determine the range to and altitude of every other aircraft.

While the system is described as using 548 transmission positions, of which 199 are used for information pulses and 248 for synchronization purposes, it should be realized that other transmission position rates, either higher or lower, may be used. Also, the rates of the information and synchronization positions may be different, if desired. The choice of the proper rates depends upon a number of factors, including station density (number of stations operating in a given area), desired range of operation between stations, system power, etc.

To summarize the various pulses that are transmitted by the system, at the start of each transmission interval each station, fixed or movable, transmits a start pulse B at position #1; information pulses G and A are transmitted by the base and movable stations respectively at the respective even and odd positions #2 to #200; and interrogation pulses I are transmitted by both the fixed and movable stations from positions #201 to #548 for synchronization purposes.

During the time allotted for synchronization (positions #201 to #548), in addition to the above pulses each station transmits .a reply pulse R (not shown in FIG. '1) under certain conditions. The reply pulse is transmitted by a station in the time interval between the transmission position at which the station transmitted its own interrogation pulse and the next transmission position. A reply pulse is also transmitted by a station only after the first interrogation pulse from another station is received and the reply pulse is used by the other station to achieve synchronization.

Thus, during the time allotted for synchronization each station transmits its own I pulses and receives I pulses from other stations. Each station also transmits R pulses in reply to certain received I pulses and receives R pulses transmitted from other stations in response to the first mentioned stations own transmitted I pulses. As is described below, each station uses its own I pulses and the R pulses received from other stations in response to its own I pulses to achieve synchronization.

III. SYNCHRONIZATION OF TWO STATIONS To explain how the many stations operating in an overall field (i.e. all those stations, fixed or movable, operating together to achieve common synchronization) of stations are synchronized, reference is made to FIGURE 2 which shows the pulse transmissions which occur when two stations in the field are synchronized. The term synchronized may be explained as follows. Starting with the condition that a pulse is transmitted by each station at the instant that an oscillator at the station which controls the pulse producing means has a certain phase angle, for example, at the time of a positive zero crossing of a sine wave, two such stations and their respective oscillators are considered to be synchronized when the pulses transmitted by each station at a particular instant are observed simultaneously at a pointmidway between the two stations. The condition of synchronization to be described holds true for two movable stations, a fixed and a movable station, or two fixed stations. It should be recalled that the stations operate to achieve synchronization only during positions #200 to #548. However, once the synchronized condition is obtained by a station, it is held during the other positions #1 to #199 by various circuits at the station.

In order to obtain synchronization between the stations during transmission positions #200 to #548, each station transmits two kinds of pulses which are respectively interrogation pulses I and reply pulses R. Each station transmits interrogation pulses I in some of the positions after #200 and reply pulses R in response to the reception of the first interrogation pulse I from another (second) station received after transmission of the first stations own I pulse. The first I pulse received would be from the closest station in the field of stations which happened to transmit at the same position.

In order to achieve the synchronized condition each station attempts to synchronize the transmission of its own interrogation pulse I with the interrogation pulse transmitted by the other station. Each station does this by determining its range to the other station. This is ac complished by having each station measure the elapsed time between transmission of the stations own interrogation pulse and reception of the reply pulses transmitted by the second station in response to this interrogation pulse. Since the round trip time between the transmission of the interrogation pulse by a station and the reception of the reply pulse transmitted by the other station in reply to this interrogation pulse multiplied by the velocity of propagation of the signal is equal to twice the range between the two stations, the actual range between the two stations can be determined by dividing the overall round trip range by two.

By measuring the range between stations, each station is also able to determine the actual transmission time of the interrogation pulse I received from the other station. This is done by determining the time of receipt of the other stations interrogation pulse with respect to the first stations own interrogation pulse transmission time and the receipt of the reply pulse from the other station. If a stations own interrogation pulse was not transmitted in synchronism with the interrogation pulse of the other station, a correction is made to the stations master oscillator so that the system pulse transmission time is corrected on subsequent transmissions. It should be pointed out that both stations are normally trying to achieve synchronism so that both will be simultaneously making corrections.

FIGURE'Z shows the synchronized condition for two stations X and Y. Lines a and b show the pulses transmitted by Stations X and Y respectively while lines c and d show the pulses respectively received by Stations X and Y. At time t=t in the in-sync (synchronized) condition, both stations X' and Y transmit the respective interrogation pulses I and I (lines a and b). This transmission would occur at any of positions #201 to #548. Pulses I and I are received at the respective Stations X and Y at time t=t The time between transmission of I (or I and reception of I (or I is called T and may be measured as a voltage E In response to the reception of the interrogation pulses at time t=t Station X transmits reply pulse R and Station Y transmits reply pulse R (lines a and b). Reply pulses R and R are received at the respective Station X and Y at time f=t (lines 0 and d). The time between transmission of I (or ly) and reception of R (or R is called T and may be measured as a voltage E or as a number of pulses counted from a clock.

For explanatory purposes, line e shows that Station X 

1. A SYSTEM FOR SYNCHRONIZING THE PRODUCTION OF FIRST SIGNALS AT TWO STATIONS COMPRISING AT EACH STATION, MEANS FOR PRODUCING SAID FIRST SIGNALS, MEANS FOR MEASURING THE DIFFERENCE IN TIME BETWEEN THE PRODUCTION OF SAID FIRST SIGNALS AT EACH STATION, AND MEANS RESPONSIVE TO A MEASURED DIFFERENCE IN TIME FOR CHANGING THE TIME OF PRODUCTION OF SAID FIRST SIGNALS TO A TIME APPROXIMATELY HALF WAY BETWEEN THE TIME DIFFERENCE MEASURED.
 45. A METHOD OF SYNCHRONIZING THE PRODUCTION OF FIRST SIGNALS AT ALL STATIONS OF A GROUP OF STATIONS COMPRISING THE STEPS OF: SYNCHRONIZING THE PRODUCTION OF FIRST SIGNALS AT ANY PAIR OF STATIONS IN THE GROUP, AND SUCCESSIVELY SYNCHRONIZING THE PRODUCTION OF THE FIRST SIGNALS OF OTHER PAIRS OF STATIONS IN THE GROUP OF STATIONS. 